1. Field of the Invention
The present invention generally relates to an apparatus and a method for reducing downhole surge pressure, for example, while running a liner into a wellbore. More particularly, the invention relates to an apparatus and a method for reducing surge pressure by actively motivating fluid flow through a tool and into an annulus exterior to the tool.
2. Description of the Related Art
Running tools are used for various purposes during well drilling and completion operations. For example, a running tool is typically used to set a liner hanger in a well bore. The running tool is made up in the drill pipe or tubing string between the liner hanger and the drill pipe or tubing string running to the surface. In one aspect, the running tool serves as a link to transmit torque to the liner hanger to help place and secure the liner in the well bore. In addition, the tool also provides a conduit for fluids such as hydraulic fluids, cement and the like. Upon positioning of the liner hanger at a desired location in the well bore, the running tool is manipulated from the surface to effect release of the liner hanger from the running tool. The liner may then optionally be cemented into place in the well bore. In some cases, the cement is provided to the well bore before releasing the liner.
One problem with running tools occurs when lowering a liner hanger, for example, at a relatively rapid speed in drilling fluid. The rapid lowering of the liner hanger results in a corresponding increase or surge in the pressure generated by the fluids below the liner string. A liner hanger being lowered in to a wellbore can be analogized to a tight fitting plunger being pushed into a tubular housing. The small annular clearance between the liner and the wellbore restricts the rate at which fluid can flow though the clearance. The faster the liner is lowered, the greater the resulting pressure or surge below the liner.
The problems associated with surge pressure are exasperated when running tight clearance liners or other apparatus in the existing casing. For example, clearances between a typical liner's Outer Diameter (O.D.) and a casing's Inner Diameter (I.D.) are ½″ to ¼″. The reduced annular area in these tight clearance liner runs results in correspondingly higher surge pressures and heightened concerns over their resulting detrimental effects.
The surge pressure resulting from running a liner/casing into a wellbore has many detrimental effects. Some of these detrimental effects include 1) lost volume of drilling fluid; 2) resultant weakening and/or fracturing of the formation when the surge pressure in the wellbore exceeds the formation fracture pressure, particularly in highly permeable formations; 3) loss of cement to the formation during the cementing of the liner in the wellbore due to the weakened and, possibly, fractured formations which result from the surge pressure on those formations; and 4) differential sticking of the drill string or liner being run into a formation during oil-well operations (that is, when the surge pressure in the wellbore is higher than the formation fracture pressure, the loss of drilling fluid to the formation allows the drill string or liner to be pulled against the permeable formation downhole, thereby causing the drill string or liner to “stick” to the permeable formation).
Typically, surge pressures are minimized by decreasing the running speed of the drill string or liner downhole to maintain the surge pressures at acceptable levels. An acceptable level is where the drilling fluid pressure, including the surge pressure, is less than the formation fracture pressure. However, decreasing running speed increases the time required to complete the liner placement, resulting in a potentially substantial economic loss.
Existing solutions to the surge pressure problem are passive in nature. In one embodiment, fluid is permitted to flow into the liner/casing and then up to the surface of the wellbore via the drill pipe. This approach is undesirable because the pressure drop through the drill pipe from the top of the liner/casing to the surface is significant, and the surge pressure below the liner/casing will still limit the run-in speed in many cases. An additional drawback is that the fluid must then be returned to the wellbore by means of some pumping facility. Another approach allows fluid flow from the interior of the liner/casing back into the wellbore via an opening formed in a tool configured as a part of the drill pipe just above the liner/casing. Such approaches are termed “passive” in that fluid flow is motivated by the lowering of the liner and associated drill pipe or tubing string. Accordingly, a surge pressure is still present and, in fact, is required to motivate fluid flow. Further, even though the pressure is being relieved, the surge pressure still increases with increasing running speeds.
Therefore, a surge reduction/elimination tool is needed which allows greater control over the surge pressure.